Re: [Qemu-devel] Re: [RFC][PATCH] make sure disk writes actually hit disk

[Jens Axboe <qemu@kernel.dk>]

On Fri, Jul 28 2006, Rik van Riel wrote:
> Anthony Liguori wrote:
> 
> >Right now Fabrice is working on rewriting the block API to be
> >asynchronous.  There's been quite a lot of discussion about why using
> >threads isn't a good idea for this
> 
> Agreed, AIO is the way to go in the long run.
> 
> >With a proper async API, is there any reason why we would want this to be
> >tunable?  I don't think there's much of a benefit of prematurely claiming
> >a write is complete especially once the SCSI emulation can support
> >multiple simultaneous requests.
> 
> You're right.  This O_SYNC bandaid should probably stay in place
> to prevent data corruption, until the AIO framework is ready to
> be used.

O_SYNC is horrible, it'll totally kill performance. QEMU is basically
just a write cache enabled disk and it supports disk flushes as well. So
essentially it's the OS on top of QEMU that needs to take care for
flushing data out, like using barriers on the file system and
propagating fsync() properly down.

-- 
Jens Axboe